1. Field of the Invention
The present invention relates generally to plasma ion deposition processes and, more particularly, to a plasma ion deposition process of large-grain, thin semiconductor films directly on low-cost amorphous substrates.
2. The Prior Art
For the large scale photovoltaic conversion of sunlight into electric power, low-cost yet high efficiency solar cells are required. Most solar cells today are made from single-crystal silicon (Si) material. These single-crystal silicon materials are anything but low-cost, however. Typically, such single-crystal silicon materials are grown in furnances by long, high temperature annealing cycles of thin small-grain polycrystalline films. Further, thin small-grain polycrystalline silicon films also are converted to large-grain polycrystalline films by propagating a melt in the film, followed by its recrystallization. Propagation of the melt is effected by laser or electron beam scanning. A still further method involves the epitaxial growing of large-grain silicon films on the texture-etched surfaces of single-crystal silicon wafers. Each of these processes produces suitable materials for high efficiency solar cells but at excessive cost.
To reduce cost, various silicon film deposition techniques also have been explored. These include the chemical vapor deposition (CVD) and the low-pressure chemical vapor deposition (LPCVD) of poly-crystalline silicon films on silicon-dioxide layers. The grain sizes were, for the most part, unsuitable for high efficiency solar cells. The glow discharge plasma deposition and the RF discharge plasma technique have yielded but amorphous silicon materials. The recently perfected technique of evaporating silicon and dopant substances in separate crucibles by electron beam heating onto high temperature substrates results in satisfactory grain size (about ten to twenty microns, with a columnar structure) but also at high cost. The economical large scale manufacture of large-grain, thin silicon films suitable for making high efficiency solar cells therefrom has remained thus far a desirable but elusive objective.